Name: chemical formula mass and percent composition
Uploaded: 2020-09-03T13:38:26.000Z
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Chemistry

Matter and its Interactions

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Formula Mass and Mole Concepts of Compounds

La plus petite unité représentative d'un composé est la molécule représentée par une formule chimique. Les masses atomiques de tous les atomes de la formule chimique, si elles sont ajoutées, donnent la masse moléculaire du composé, mesurée en unité de masse atomique. Par exemple, pour trouver la masse moléculaire de saccharose ou sucre de table, la masse atomique de chaque élément est multiplié par le nombre d'atomes de l'élément correspondant.La somme totale donne la masse moléculaire du saccharose. La masse de toutes les molécules présentes dans une mole d'un composé chimique est denommé sa masse molaire, mesurée en grammes par mole. La masse molaire d'un composé est numériquement égale à sa masse moléculaire Par exemple, le saccharose, avec une masse moléculaire de 342, 3 unité de masse atomique, aura une masse molaire de 342, 3 grammes par mole.Connaître le nombre réel de molécules ou de particules présentes dans une substance chimique est fondamental pour comprendre sa composition, ses interactions et ses combinaisons. Cependant, dans un échantillon d'un composé, comptant le nombre réel de molécules par des moyens ordinaires est impraticable. En utilisant la relation entre la masse molaire, la mole et le nombre d'Avogadro, les molécules totales présentes peuvent être calculées.Selon le concept, la masse molaire d'un composé est équivalente à une mole du composé, constituant des particules 6, 022 x 10^23. Par conséquent, 342, 3 grammes de saccharose contiennent 6, 022 x 10^23des molécules de saccharose. Cependant, si un échantillon de saccharose pèse 150 grammes par exemple, alors le nombre de molécules peut être déterminé par calculer en utilisant une méthode de multiplication croisée.Le résultat montre que 150 grammes de saccharose constituent 2, 63 x 10^23 des molécules de saccharose. Le pourcentage de chaque élément dans un composé peut être exprimé en utilisant une valeur en pourcentage de masse. Pour calculer, la masse de l'élément dans une mole du composé est divisée par la masse totale d'une mole du composé et convertie en un pourcentage.En pratique, pour calculer le pourcentage massique d'hydrogène dans le saccharose, la masse totale des atomes d'hydrogène et la masse totale des molécules de saccharose sont incluses dans la formule. La valeur calculée montre que le saccharose contient 6, 4%en masse d'hydrogène.

formule de masse et concept des moles

<h4>Masse de la formule des composés covalents </h4>
 
 Les formules chimiques représentent la composition élémentaire des substances. Pour les composés covalents, la formule représente les nombres et les types d'atomes composant une seule molécule de la substance ; par conséquent, la masse de la formule peut être correctement appelée masse moléculaire. La formule moléculaire du chloroforme (CHCl3), un composé covalent, indique qu'une seule molécule contient un atome de carbone, un atome d'hydrogène et trois atomes de chlore. La masse moléculaire moyenne d'une molécule de chloroforme est égale à la somme des masses atomiques moyennes de ces atomes : 
 
 <img alt="Eq1" src="https://www.jovecore.com/CDNSource/lessons/1017/1017_Equations-01.png" style="height: 40px;" />
 
 <h4>Masse de la formule des composés ioniques </h4>
 
 Les composés ioniques sont composés de cations et d'anions distincts qui sont combinés dans des rapports pour produire un gros volume de matière électriquement neutre. La masse de la formule pour un composé ionique est calculée de la même manière que la masse de la formule pour les composés covalents : en additionnant les masses atomiques moyennes de tous les atomes dans la formule du composé. Toutefois, la formule d'un composé ionique ne représente pas la composition d'une molécule distincte, elle peut donc être appelée à tord la ‘ masse moléculaire ’. 
 
 Par exemple, le sel de table commun ou le chlorure de sodium (NaCl) est un composé ionique composé de cations sodium (Na+) et d'anions chlorure (Cl−) combinés dans un rapport 1:1. La masse de la formule pour ce composé est calculée en ajoutant les masses atomiques moyennes de ses éléments constitutifs : 
 
 <img alt="Eq2" src="https://www.jovecore.com/CDNSource/lessons/1017/1017_Equations-02.png" style="height: 40px;" />
 
 Lors du calcul de la masse de la formule d'un composé ionique, on a utilisé les masses moyennes des atomes neutres de sodium et de chlore, plutôt que les masses des cations sodium et des anions chlorure. Même si un cation sodium a une masse légèrement plus petite qu'un atome de sodium (puisqu'il lui manque un électron), cette différence sera compensée par le fait qu'un anion chlorure est légèrement plus massif qu'un atome de chlore (en raison de l'électron supplémentaire). De plus, la masse d'un électron est négligeable par rapport à la masse d'un atome typique. 
 
 <h4>Composition en pourcentage massique </h4>
 
 La composition élémentaire d'un composé définit son identité chimique, et les formules chimiques sont la façon la plus concise de représenter cette composition élémentaire. Le pourcentage en masse de chaque élément dans un composé est appelé le pourcentage massique de cet élément particulier. La composition en pourcentage peut être calculée en divisant la masse de chaque élément par la masse globale du composé, puis en convertissant en pourcentage. 
 
 La composition en pourcentage est utile pour évaluer l'abondance relative d'un élément donné dans différents composés de formules connues. Tant que la formule moléculaire ou empirique du composé en question est connue, la composition en pourcentage peut être déduite des masses atomiques ou molaires des éléments du composé.  
 
 Par exemple, une molécule d'acide nitrique (HNO3) contient un atome d'azote pesant 14,01 uma, un atome d'hydrogène pesant 1,008 uma et trois atomes d'oxygène pesant (3 × 16,00 uma) = 48,00 uma. La masse de la formule de l'acide nitrique est donc (14,01 uma + 1,008 uma + 48,00 uma) = 63,02 uma, et sa composition en pourcentage est : 
 
 <img alt="Eq3" src="https://www.jovecore.com/CDNSource/lessons/1017/1017_Equations-03.png" style="height: 40px;" />
 
 <img alt="Eq4" src="https://www.jovecore.com/CDNSource/lessons/1017/1017_Equations-04.png" style="height: 40px;" />
 
 <img alt="Eq5" src="https://www.jovecore.com/CDNSource/lessons/1017/1017_Equations-05.png" style="height: 40px;" />
 
 Ce texte est adapté de <a href="https://openstax.org/books/chemistry-2e/pages/3-1-formula-mass-and-the-mole-concept">Openstax, Chimie 2e, Section 3.1: Formule de la masse et concept de mole</a> et <a href="https://openstax.org/books/chemistry-2e/pages/3-2-determining-empirical-and-molecular-formulas">Openstax, Chimie 2e, Section 3.2 : Détermination des formules empiriques et moléculaires.</a>

Formule de masse et concept des moles

Formule Masse des composés covalents

Les formules chimiques représentent la composition élémentaire des substances. Pour les composés covalents, la formule représente le ...

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chemical formula mass and percent composition

Chemical formula mass and percent composition help describe the makeup of compounds. A chemical formula shows which atoms are present and how many of each atom appear in a substance.

For covalent compounds, the formula shows the atoms in one molecule. That is why formula mass can also be called molecular mass. Chloroform, CHCl3, is one example. Its formula shows one carbon atom, one hydrogen atom, and three chlorine atoms. The molecular mass is the sum of their average atomic masses.

Ionic compounds are different because they contain cations and anions that combine in ratios that make the bulk material electrically neutral. Their formula mass is found the same way, by adding the average atomic masses of the atoms in the formula. Sodium chloride, or table salt, is one example. Its formula, NaCl, shows sodium and chloride in a 1:1 ratio. For ionic compounds, the term molecular mass is not the best choice because the formula does not describe a single molecule.

When calculating formula mass for an ionic compound, neutral atom masses are used instead of ion masses. A sodium cation has slightly less mass than a sodium atom because it has lost an electron. A chloride anion has slightly more mass than a chlorine atom because it has gained an electron. The difference is small, and the mass of an electron is negligibly small compared with the mass of a typical atom.

Percent composition tells how much of a compound’s mass comes from each element. It is found by dividing the mass of each element by the total mass of the compound and converting the result to a percentage. This is useful for comparing the relative amount of an element in compounds with known formulas.

Nitric acid, HNO3, shows how the calculation works. One molecule contains one nitrogen atom with a mass of 14.01 amu, one hydrogen atom with a mass of 1.008 amu, and three oxygen atoms with a total mass of 48.00 amu. Its formula mass is 63.02 amu. From that total, the percent composition of each element can be determined.

Chemical Formula Mass and Percent Composition

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formula mass and mole concepts of compounds

Chemical formula mass and percent composition help describe the makeup of compounds. A chemical formula shows which atoms are present and how many of each atom appear in a substance. For covalent compounds, the formula shows the atoms in one molecule. That is why formula mass can also be called molecular mass. Chloroform, CHCl3, is one example. Its formula shows one carbon atom, one hydrogen atom, and three chlorine atoms. The molecular mass is the sum of their average atomic masses. Ionic...

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Learn how to calculate formula mass for covalent and ionic compounds, then use it to find percent composition with examples like chloroform, salt, and nitric acid.

Chemical Formula Mass and Percent Composition | JoVE K12

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Chemical Formula Mass and Percent Composition

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